Next experiment was to test built amplified sensor signal’s circuit. Results weren’t stunning. Seems like it’s precise calibration requires much more time.

After it Furqan helped to test geophone’s sensitivity regarding to levels of low frequencies. Speaker was playing inaudible audio. The frequency was always the same (25hz). Even though we couldn’t hear anything it was obvious from led’s bar that decibels are going higher (test is on the video).

Interface for transmitting tones as a code.
In the background of the video you can hear street noise to which interface reacts. It detects only the lowest range of frequencies (0-80 hz). Three bars determines volume level (dB). Each bar takes 24 random tones. The white bar represents the whole range of levels.